Transistor amplifiers have a peak efficiency for a particular input power that is a function of geometry (i.e. circuit components and layout), load and supply voltage. In conventional radio frequency (RF) power amplification these characteristics are fixed based on the peak input level expected. For amplifiers presented with an input signal having a wide dynamic range, the input signal infrequently achieves peak levels and frequently operates below peak levels. As such, the amplifier may exhibit low overall efficiency.
Various techniques are known in the art for enhancing amplifier efficiency based on the supply voltage. One broad classification of solution is envelope tracking.
In a prior art envelope tracking technique, a switched mode pulse width variable modulator may be combined with a linear amplifier such that the efficient switched mode supply provides the low frequency components of the output signal that contains a majority of the required power, and the linear amplifier provides a high bandwidth signal to provide the high frequency components of the output signal and correct errors in the switched supply output. A power supply with high bandwidth and good efficiency is thereby provided.
GB 2409115 describes an example of such a prior art envelope tracking power supply. A schematic representation of the described power supply is illustrated in FIG. 1. In general, the outputs from a switch mode supply and a linear amplifier are summed in a transformer in order to provide the required voltage to a load. The power supply, generally designated by reference numeral 100, comprises a switchable main voltage source 102 coupled to a first tap 103 of a secondary winding 110 of a transformer 104. A second tap 105 of the secondary winding 110 is coupled to a load 101a. A reference voltage source 114 is provided which is coupled to a first input of a subtractor 112, the subtractor 112 having a second input coupled to the second tap 105 of the secondary winding 110. An output of the subtractor 112 is coupled to an input of a correction amplifier, or driver, 106. The output of the correction amplifier 106 is coupled to a first tap 107 of a primary winding 108 of the transformer 104. A second tap 109 of the primary winding 108 is coupled to ground.
In operation, the switchable main voltage source 102 provides a coarse voltage signal for delivery to the load. Typically this is provided by a selection of one of a plurality of voltage supplies in dependence on a reference voltage source derived from the same origin as the reference voltage source 114.
Subtractor 112 determines an error value between the required voltage, as provided by the reference voltage source 114, and the actual output voltage being provided to the load 101a. The correction amplifier 106 receives this error value and provides an error correction signal at its output which is applied to the primary winding 108 to be summed with the output of the switchable main voltage source 102 passing through the secondary winding 104.
The output voltage provided at tap 105 of the secondary winding is therefore the coarse voltage provided by the switchable main voltage source 102 combined with an error voltage provided by the subtractor/driver 112/106. The transformer 104 performs the combining. The use of a transformer to perform the combining is advantageous and desirable.
However, the coarse voltage signal output by the switchable main voltage source 102 may include a significant DC component which will flow through the secondary winding 110 of the transformer 104. DC current flowing in the transformer windings may result in saturation of the core, with a consequent loss of magnetising inductance, which is clearly undesirable.
One possible way to avoid saturation of the core due to a DC current present in one of the transformer windings is to increase the size of the core, and thereby increase the amount of magnetic flux in the core before it becomes saturated. However, increasing core size increases the transformer leakage inductance and reduces the high frequency response of the transformer and in turn, reduces the high frequency response of the power supply.
An alternative technique for avoiding saturation is to introduce an airgap in the core, but this increases the number of turns required to achieve a given low frequency performance and indirectly increases the leakage inductance, and again reduces the high frequency response of the supply.
A further method of avoiding core saturation is described in GB 2409115. The solution proposed is to inject a DC current into the primary winding 108 of the transformer 104 in such a way as to cancel the flux due to the secondary current. However, this requires control systems to monitor the currents in both the secondary and primary windings of the transformer, and is complex and difficult to control.
It is an aim of the present invention to provide an improved scheme which addresses one or more of the above-stated problems in particular it is an aim of the invention to provide a scheme for reducing DC current in a transformer used as a combiner to combine a DC and an AC signal.